Preliminary study of aromatase expression and sexual steroids level in different sexual phenotypic/genotypic combination of Nile tilapia, (Oreochromis niloticus)

Peculiar sexual phenotype/genotype combinations of O. niloticus are largely used to investigate the sex determinism mechanisms, and to produce monosex populations in fish farming. Preliminary results showed that tilapias with atypical phenotype/genotype combinations have different reproductive performances (e.g. spawning frequency, hatching rate). These differences could be linked with aggressiveness level and/or physiological particularities. This study tried to highlight differences in plasma level of sexual steroids and in the expression of brain and gonad aromatase between fish characterized by different sexual phenotype/genotype combinations. Fish were held in 1.6 m³ tanks (4 males, 14 females) with different crosses: ♂XY × ♀ YY; ♂YY × ♀ XX; ♂XX × ♀ XX; ♂XY × ♀ XX. After 10 days, blood, brain and gonads were sampled on 60 fish (BW = 447 ± 105 g). Plasma level of 17β-oestradiol (E2), testosterone (T) and 11-ketotestosterone (11KT) were measured by RIA and RT-QPCR was used to assess the expression level of gonad (AromA) and brain (AromB) aromatase in the brain and the gonads. Mean relative expression of AromA was about 50 times higher in female (between 43.3 ± 32.2 and 72.8 ± 54.0) than in male (between 0.6 ± 0.5 and 2.3 ± 2.3) gonads but was not significantly different between genotypes of the same phenotypic sex. AromA was not expressed in the brain. Expression of AromB was lower but was found in the brain (from 11.7 ± 19.8 to 13.1 ± 14.4 in females; from 1.0 ± 1.0 to 10.9 ± 13.7 in males) and gonads (from 1.9 ± 1.6 to 3.9 ± 2.0 in females; from 0.5 ± 0.2 to 1.0 ± 1.1 in males) of both sexes. Plasma concentrations of T and 11KT were higher in males (3.3 ± 5.6 ng/ml, 10.9 ± 10.6 ng/ml respectively) than in females (0.7 ± 1.6 ng/ml, 1.3 ± 1.1 ng/ml respectively) and E2 concentration was higher in females (4.0 ± 4.1 ng/ml) than in males (2.0 ± 3.1 ng/ml). Within a same phenotype, no statistical difference could be highlighted between the different genotypes. In this study, endocrinological differences between groups seem to be related only with phenotype but not with genotype. However, genotypic differences could be hidden by the great interindividual variability. Therefore, these preliminary results need to be refined by using larger number of fish and better synchronizing fish maturation.